Modulated signals, carrying information such as video, data, music and speech are generally contaminated by noise. Efficient demodulation requires distinguishing the information from the noise.
The demodulation process includes several steps. The receiver may receive, at its antenna, an information signal modulated on a radio frequency (RF) carrier. The signal may then undergo frequency conversion to the intermediate frequency (IF) band. The information signal, at baseband, is recovered from the IF signal by a suitable detector. Considering, for example, a conventional receiver in a variable rate digital data transmission system, the IF signal, produced from a received RF signal by subjecting the RF signal to a mixing or filtering process, is subsequently applied to a data detector for recovering, at baseband, the information content of the input signal. As the system must be responsive to a variable rate signal, the IF bandwidth must be broad enough to process the highest expected data rate, although at any point in time the receiver may be detecting a lower rate and thus narrower band signal. As the noise bandwidth is not limited to the frequency spectrum, that is, bandwidth, of the received signals, the bandwidth of the receiver'front-end, that is, prior to detection, must be scaled with the received signalling rate to prevent noise overload, signal suppression, and distortion in subsequent digital processing stages. To effect this scaling it is conventional to use some type of filter switching mechanism limiting the IF bandwidth based on the received signalling rate.
A conventional filter switching arrangement for limiting the noise bandwidth at a receiver front end is illustrated in FIG. 1. This arrangement may be used in a receiver of a digital data transmission system to select a bandwidth at IF sufficient to pass data signals transmitted at a selected one of several data rates, while suppressing noise outside that bandwidth.
The FIG. 1 arrangement includes an input terminal 6 receiving the incoming modulated signal and noise at IF. The input terminal 6 is connected to a commutator 4 of a rotary switch 2. The switch 2 has a number of fixed contacts 8.sub.1 --8.sub.n each selectively connected to the commutator 4 through rotation of the commutator. Each fixed contact 8.sub.l' --8.sub.n is electrically connected to a respective IF filter 10.sub.l --10.sub.n. The center frequencies F.sub.l --F.sub.n and bandwidths BW.sub.l --BW.sub.n of the IF filters 10.sub.l --10.sub.n are selected on the basis of the data rates the receiver is designed to accept. The outputs from the IF filters are input to a power combiner 12. The output from the power combiner is an IF signal whose bandwidth is scaled to the signalling rate of the received signal, that is, somewhat greater than, but proportional to, the bandwidth of the received data or symbol rate, thereby reducing the noise bandwidth prior to data detection in a detector 14. The reduced noise bandwidth prevents noise overload, signal suppression and distortion in the latter processing stages of the detector 14.
More specifically, in operation of the conventional arrangement of FIG. 1,an RF signal, modulated by a data signal at the selected symbol rate, is converted to IF by conventional mixing or filtering and then applied to input terminal 6. One of the parallel sets of filter paths is selected by rotating commutator 4 based on the symbol rate of the data signal modulating the IF signal. The selected one of the IF filters 10.sub.l --10.sub.n limits the bandwidth of the IF signal prior to detection, thereby reducing the noise bandwidth which initially extends over the entire IF spectrum. This conventional arrangement suffers from several disadvantages. For example, it is expensive and cumbersome to implement, and it produces gain and phase variations from one path to another as well as from one unit to another.
If the filter responses are relatively simple, a single filter implementation with switched elements might be used instead of the plural paths of the filters. However, even in this case, the disadvantages stated above exist.
Boxcar filtering is another technique which might be used to reduce pre-detection noise. Boxcar filtering involves averaging the incoming signal, with noise reduction the expected result since noise is theoretically random. Over a period of time many random signals have substantially equal positive and negative components, and thus averaging will tend to reduce the noise component of such a signal toward zero. However, note that boxcar filtering is not applicable to digital data demodulation since with the boxcar technique averaging must be done over many symbols and the exact period of the signal to be averaged must be known.
The present invention is directed to a technique and implementing apparatus which do not experience the aforementioned disadvantages of either the conventional bandwidth switching technique or boxcar filtering technique.